This condition is more common than you think with all castings. None are porosity free, but every now and then it will show to the outside as a leak if in the right place. I weld aluminum all the time, and it would be my last choice to fix it. Firstly it anneals the parent metal making the zone softer than the surrounding area. Then there's the distortion problem. With welding comes distortion, no way around it. While it maybe only .002 in that spot, that's huge on a cyl deck. My proceedure with wicking grade loctite is simple and permanent.
Whipe all oil off with brake cleaner until spotless. Heat suspect area with a heat gun. This will expand any fluids trapped in the porosity and will ooze out showing the location of the leak. Mark the area with a marker and continue to heat and blow until no more oil appears. Let it cool. Clean again with brake cleaner and brush the #292 over the problem zone. The object is to keep the area wet and allow the loctite to creep back into the voids. Forcing the fluid back and forth with your finger will jam it in deeper, but not always possible depending on location. A small paint brush, going back and forth works well too. After about 15 minutes whipe off the excess and apply light heat from a heat gun to expand and cure the loctite. For me, this has been a 100% success rate on many applications.
A few years ago we machined a huge water pump for a US nuclear plant. On the 200 psi water test a small leak was pissing a stream of water out. Repeated attempts of welding this huge casting failed. The only other approved method was loctite and it was an instant success. That was 15 years ago. Sleep tight.:redrolf:
Ron